1. Field of the Invention
This invention relates generally to a method for controlling a regenerative braking event in a hybrid electric vehicle wherein blend-out control is coordinated with torque converter control.
2. Description of the Prior Art
In order to maximize fuel economy in a hybrid electric vehicle, energy saving through regenerative braking with the electrical motor during a brake event is an integral part of the system. A high voltage battery can be charged through regenerative braking while the vehicle is slowing down. On the other hand, to maintain vehicle safety, conventional friction braking is also used when regenerative braking by itself cannot meet the driver demand. At low speed before the regenerative braking becomes unavailable or inefficient; regenerative braking torque should be ramped down to zero gradually and the conventional friction braking torque should ramped up to the driver demand to facilitate smooth brake torque transition. This process is referred as regenerative braking torque blend-out. Thus the driver brake demand can be satisfied with acceptable vehicle drivability, fuel efficiency and vehicle safety.
In a modular hybrid electric vehicle, a step ratio transmission and torque converter are located between an electrical motor and the driving axle with wheels. During a regenerative braking event, through the transmission and the torque converter the vehicle kinetic energy charges the high voltage battery by rotating the electrical machine. The torque converter clutch must be locked in order to fully transmit the vehicle kinetic energy to the electrical motor. If the torque converter must be open, it would transmit much less vehicle kinetic energy. In this case, the regenerative braking torque blend-out should be completed before the torque converter opens to prevent stalling the electrical motor.